at least one volume could not be unmounted

If you specify a partition, then that partition's whole-disk parent is assumed. Barrys-Mac-mini:~ barryhill$ sudo diskutil unmountDisk force /dev/disk2 Password: Forced unmount of disk2 failed: at least one volume could not be unmounted Barrys-Mac-mini:~ barryhill$ sudo diskutil unmount force /dev/disk2 disk2 was already unmounted or it has a partitioning scheme so use 'diskutil unmountDisk' instead Barrys-Mac-mini:~ barryhill$ diskutil unmountDisk Usage: diskutil umountDisk MountPoint|DiskIdentifier|DeviceNode Attempt to unmount the directly-mountable volumes on all of a whole disk's partitions as well as all virtual volumes which are implied by storage systems such as AppleRAID, CoreStorage, and APFS. This will delete your CoreStorage volume and reformat it as an unencrypted HFS+ volume.10.15.4 I have a third external drive, lacie 1tb, which works fine on my other mac mini, but that won't mount either. The LVG should be the first UUID listed and it’s the one we want to delete. In the listing, you will want to select and copy the Logical Volume Group (LVG) alphanumeric UUID for your CoreStorage volume. Unless you have a Fusion drive or multiple encrypted drives, your FileVault 2-encrypted drive should be the only one listed. This will give you with a list of the CoreStorage volumes on your system. To do this, first run the following command: This includes the ability to erase encrypted CoreStorage volumes (aka FileVault 2-encrypted volumes) without first decrypting or unlocking them. To help with this, the diskutil tool provides a way to quickly delete CoreStorage volumes.

This can be an issue for a malfunctioning FileVault 2-encrypted volume that will not let you either unlock or decrypt.

However, Disk Utility won’t let you erase or repartition until you unlock or decrypt. On occasion, it’s necessary to erase a FileVault 2-encrypted volume.

At least one volume could not be unmounted